ENV/std: restore original make_jobs return type

[Bo98]

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This previously returned an integer and there is code which relies on this. (https://jenkins.brew.sh/job/Homebrew%20Core%20Pull%20Requests/60693/version=mojave/console)

The superenv version of make_jobs worked differently and still returns an integer.

The superenv-specific determine_make_jobs is probably not a concern. It could potentially even be removed entirely and its implementation inlined.

[MikeMcQuaid]

Thanks @Bo98!